The present invention is generally directed to a system and process for cleaning forming fabrics during a wet papermaking process. More particularly, the present invention is directed to a system and process for removing unwanted fibers from a forming fabric by contacting the fabric with a tangential fluid spray.
In making various paper products, typically an aqueous suspension containing pulp fibers is first formed. The aqueous suspension is then spread out over a forming surface in order to form a paper web. The forming surface generally includes a series of endless conveyors which are formed from a porous fabric that can be made from metal, plastic, or any other suitable material. The forming fabrics are designed to facilitate formation of the nonwoven web, to transport the nonwoven web, and to remove excess liquid from the web as it travels downstream.
From the forming fabric, the nonwoven web is usually transported through a press section and then through one or more driers. Depending upon the paper product being formed, the nonwoven web can then be subjected to various post formation processes as desired.
One problem that is typically encountered during the formation of paper products is that the forming fabrics have a tendency to become fouled and clogged by bonding materials, additives contained within the fiber suspension, and especially, by paper fibers, which are referred to as xe2x80x9cfiber carry backxe2x80x9d. Too much debris and fiber carry back on the forming fabric can create fiber waste and also can adversely affect sheet formation. The problems with fiber carry back become especially severe when the sheet being formed has a relatively low basis weight such as when making tissue paper, when short fibers at a low consistency are being used to form the paper sheet, at higher machines speeds, and when excessive amounts of fiber carry back begin to accumulate on the forming fabrics.
In the past, various attempts have been made in order to remove debris and contamination from the forming fabric. Systems used in the past, however, have generally not been very efficient at removing especially paper fibers. For example, although the typical high pressure water shower spraying onto the sheet side of the fabric can clean the majority of the fiber carry back, it also can push the fibers deeper into the interstices of the woven fabric causing accumulations that are much more difficult to remove.
As such, a need currently exists for a system and process that is capable of more effectively cleaning a forming fabric in a wet papermaking process. In particular, a need exists for a system and process for removing fibers, such as paper fibers, from a forming fabric that is used to transport a web prior to drying. A need further exists for a process and system for cleaning a forming fabric that does not substantially hinder or obstruct movement of the fabric.
Accordingly, it is object of the present invention to provide a process and system for cleaning debris, especially fibers, from a forming fabric in a wet papermaking process.
Another object of the present invention is to provide a system and process for cleaning a forming fabric in a papermaking process in which a stream of fluid tangentially contacts the sheet side of the forming fabric for removing the carry back fibers and other debris from the fabric.
Still another object of the present invention is to provide a process and system for cleaning a forming fabric that does not interfere or hinder the fabric during formation of a paper web.
These and other objects of the present invention are achieved by providing, in one embodiment, a system for removing unwanted fibers from a forming fabric during a wet papermaking process. The system includes a porous forming fabric that can be an endless conveyor made from, for instance, metal, plastic or any other suitable material. The forming fabric is positioned to receive a nonwoven web formed from an aqueous suspension of fibers. After the nonwoven web is transferred to the next process station, such as a press or a drying section, the forming fabric may be wrapped around one or more turning rolls as it is routed back to the forming section.
In accordance with the present invention, in order to remove fibers and other debris from the forming fabric, the system further includes a scarfing shower that is configured to emit a stream of fluid. In particular, the scarfing shower is positioned such that the stream of fluid tangentially contacts the forming fabric while the forming fabric is moving around the turning roll. Preferably the forming fabric is wrapped around the turning roll less than 90 degrees, and particularly less than 120 degrees, such as from about 2 degrees to about 120 degrees. For example, the scarfing shower can include one or more rows of nozzles that are spaced along the width of the forming fabric. Each nozzle can emit a cleaning fluid that slightly impinges and is mostly tangent to the fabric.
The fluid that is emitted by the scarfing fan shower preferably covers 100% of the fabric width to be cleaned. The shower can be, for instance, a liquid such as water. In one embodiment, the cleaning fluid can be a mixture of water and a surfactant, such as a soap. The liquid can be emitted from the scarfing shower at a pressure of at least 50 psi, and particularly at a pressure of at least 100 psi. In one embodiment, the cleaning fluid can be emitted from the scarfing shower at a pressure of from 100 psi to about 400 psi.
As stated above, the scarfing shower is positioned so as to contact the forming fabric with a fluid at a tangent to the fabric as the fabric is moving around the turning roll. Preferably, the direction of the cleaning fluid is opposite to the direction at which the forming fabric is moving. In this manner the relative velocity of the shower liquid to the fabric is enhanced and the cleaning power is also enhanced. In order to achieve this result, the scarfing shower can be positioned at any suitable location. For most applications, however, the scarfing shower contains a row of nozzles spaced from about 0.5 inches to about 3 inches apart, and particularly from about 1 inch to about 2 inches apart. Further, the nozzles can be positioned such that the issued liquid travels from about 2 inches to about 6 inches to hit the fabric, and particularly travels from about 3 inches to about 4 inches to hit the fabric.
In order to collect the cleaning fluid after it impinges upon the forming fabric, the system can also include a collecting pan that surrounds the area of the fabric that is cleaned. The collecting pan can include a drain for removing the liquid as it is collected.
These and other objects of the present invention are also achieved by providing a process for cleaning a forming fabric in a wet papermaking process. The process includes the steps of forming an aqueous suspension of fibers that are spread over a forming fabric in order to form a nonwoven web. The forming fabric can be, for instance, an endless belt that is wrapped around a turning roll at least 90 degrees. Once formed, the nonwoven web is removed from the forming fabric prior to the turning roll.
In accordance with the present invention, while the forming fabric is moving around the turning roll, the fabric is contacted with a stream of liquid at a location that is generally tangent to the fabric. The stream of liquid contacts the fabric in a direction that is generally opposite to the direction of the forming fabric, which removes fibers and other debris from the fabric as the fabric is moving through the system.
Other objects, features and aspects of the present invention are discussed in greater detail below.